Transverse fans are also known as cross-flow and tangential fans. They are used in air conditioning applications because of their in-line flow capabilities and their suitable relationship with plate-fin heat exchangers since they can extend the entire length of a heat exchanger. To achieve the desired length, the impeller can be made up of a plurality of segments or modules with one or more segments being shorter than the others in order to achieve the total desired length. In a transverse fan, the inlet and outlet are, generally, nominally, at right angles but angles from 0 to 180.degree. are possible. The impeller is similar to a forward curved centrifugal fan wheel except that it is closed at both ends. The flow is perpendicular to the impeller axis throughout the fan, and enters the blade row in the radially inward direction on the upstream side, passing through the interior of the impeller, and then flowing radially outward through the blading a second time. The flow is characterized by the formation of an eccentric vortex that runs parallel to the rotor axis and which rotates in the same direction as the rotor.
A two stage action occurs as the flow passes first through the suction (upstream) blading and then through the discharge blades. The flow contracts as it moves across the impeller producing high velocities at the discharge blades (second stage). The flow leaves the impeller and contracts again as it turns and squeezes around the vortex. The combination of these effects results in the high pressure coefficients attained by transverse fans. A vortex wall separates the inlet from the outlet and acts to stabilize the vortex. Since there is only re-circulating flow in the region of the vortex, no useful work is done there. The main effect in the vortex is energy dissipation. Fan stability is, however, highly sensitive to vortex wall clearance. This parameter must be controlled very carefully since a trade-off is made between stable, high performance and tone noise generated by interaction of the impeller with the vortex wall. The vortex wall coacts with the blades of the impeller as they move from the discharge side to the suction side. In a high wall indoor fan coil unit of a duct-free split system a noise problem existed caused by unstable flow due to flow separation from the rear/bottom wall, particularly near the two end walls. It is speculated that a vortex, or flow separation, was being established on the rear/bottom wall.